The 2018 Hokkaido Eastern Iburi Earthquake andHidaka arc-arc collision system

Editorial Board (November 2021)
Editor-in-Chief
Takeshi Sagiya, Nagoya University, Japan

Vice Editors-in-Chief
Masahito Nosé, Nagoya University, Japan

Aitaro Kato, University of Tokyo, Japan

Editors
Hideo Aochi
Pascal Audet
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Hao Dong
Guangyu Fu
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Mie Ichihara
Takeo Ito
Hidekatsu Jin
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Tomokazu Kobayashi
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Advisory Board
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Toshihiko Iyemori

Journal Scope
Earth, Planets and Space (EPS) is the official journal of Society of Geomagnetism and Earth, Planetary
and Space Sciences, The Seismological Society of Japan, The Volcanological Society of Japan, The
Geodetic Society of Japan, and The Japanese Society for Planetary Sciences.
EPS is a peer-reviewed, open-access journal published under SpringerOpen. It is an international journal
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Earth, Planets and Space(2020) 72:65
Takahashi et al. Earth, Planets and Space
https://doi.org/10.1186/s40623-020-01176-4

Open Access

PREFACESpecial issue “The 2018 Hokkaido Eastern
Iburi Earthquake and Hidaka arc–arc collisionsystem”
Hiroaki Takahashi1*, Nobuo Takai2, Masahiro Chigira3, Guojie Meng4, Saeko Kita5 and Takuji Yamada6

local seismic network data with a 3-D velocity structure.
They also suggested that the aftershock area consistedof three segments, which reflects a complex rupture
process. The aftershock distribution and the estimatedfault model were not consistent with the configurationof a nearby major active fault (the Ishikari–Teichi–Toenfault). Ohtani and Imanishi (2019) and Kobayashi (2019a,b) assessed the possible build-up of stress on this adjoining major active fault owing to co- and post-seismic
crustal deformation, and the statistical characteristics ofthe seismic activity before and after the mainshock were
investigated by Kumazawa et al. (2019) using the ETAS
model.
The strong ground motions due to this event were accurately recorded by the dense nationwide seismographic
network. Dhakal et al. (2019) found large PGAs and
PGVs at a medium distance relative to the standard distance attenuation model. Nakano and Kawase (2019) suggested that the observed stress drop corresponded to theupper limit of a crustal earthquake. Dhakal et al. (2019)
and Nakano and Kawase (2019) also reported nonlinear
site amplification at seismic stations near the epicenter.
Takai et al. (2019) concluded that the destructive strong
shaking observed in the town of Mukawa resulted fromthe local and shallow underground structure based on
subsurface structure surveys. The seismic source properties and amplification characteristics described herein
provide insights that can lead to the better evaluation ofstrong ground motions.
The regional subsurface structure within the Hidaka
collision system was investigated using seismic andmagnetotelluric data. Iwasaki et al. (2019) illustrated
that the Hidaka collision zone consists of a complex

The 2018 Hokkaido Iburi Eastern Earthquake on 9 September 2018 was an inland moderate-sized Mw6.6
earthquake. The maximum intensity of VII, which is thehighest in JMA intensity scale, was observed in the vicinity of the epicenter. This earthquake resulted in over 40
deaths, most of which were caused by enormous seismically induced landslides. Strong shaking also destroyedelectric power facilities, causing a long-term electrical
power outage throughout the Hokkaido region. This special issue focuses on describing this unique earthquakeand discussing the characteristics of the seismic source
process, strong ground motions, aftershock activity, tectonic background, and other related phenomena.
The source process and fault model of the earthquake
have been reported in the following papers. Kobayashi
et al. (2019a) suggested the steeply dipping rectangular fault at depths of 15–30 km using GNSS and InSAR
geodetic data. Kobayashi et al. (2019b), Asano and Iwata(2019), and Kubo et al. (2020) discussed the rupture process in detail using strong motion data and suggested theexistence of major slip at depths of 20–30 km, approximately 10 km shallower than the hypocenter. Additionally, Asano and Iwata (2019) simulated the seismic
wavefield using a 3-D velocity model to reflect the complex subsurface structure of the focal region.
Katsumata et al. (2019) indicated that the rupture initiated in the deepest part of the aftershock region using*Correspondence: hiroaki@sci.hokudai.ac.jp1
Institute of Seismology and Volcanology, Faculty of Science, Hokkaido
University, Sapporo, Japan
Full list of author information is available at the end of the article© The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing,
adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) andthe source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material
in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material
is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds thepermitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativeco
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The 2018 Hokkaido Eastern Iburi Earthquake and Hidaka arc-arc collision system
Takahashi et al. Earth, Planets and Space(2020) 72:65

Page 2 of 3

crustal and upper mantle structure using controlled
source seismic data and suggested that the mainshock
and a number of aftershocks occurred in the upper
mantle. Kita (2019) indicated that the relocated aftershocks determined by a regional seismic network
were situated in an anomalously thick crust and were
distributed near the boundaries of velocity and Qp
attenuation structures. Nakamura and Shiina (2019)
also suggested that a Qs boundary corresponded to theaftershock alignment. Ichihara et al. (2019) presented
the heterogeneous electrical resistivity structure of the
Hidaka collision zone and suggested a possible conductive zone in the aftershock region. Ikeda and Takagi(2019) observed a coseismic velocity reduction as wellas possible crack formation in the focal region. Iwasaki
et al. (2019) and Kita (2019) suggested that low heatflow was the cause of the anomalously deep rupture.
Ohzono et al. (2019) revealed a relatively high strain
rate in the focal region using GNSS data and suggested that the presence of weak crustal material in thefold–thrust zone leads to strain accumulation. Ito et al.
(2019) illustrated the possible convergence of tectonic
blocks in northern Hokkaido from regional GNSS data,
where the northern continuation of the focal region is
located. These geodetic data suggest the regional andconstant build-up of stress in central Hokkaido.
The topographic characteristics of seismicallyinduced landslides were assessed by Kasai and Yamada (2019), who indicated that the slip surfaces ofnumerous shallow landslides formed in the remainsof volcanic soil deposited 9000 year BP. The landslides
resulting from this earthquake were more clustered,more numerous, and larger in scale than were expectedfrom the magnitude of the earthquake. Fujiwara et al.
(2019) reported linear surface ruptures with small displacements from InSAR images; they also extractedliquefaction areas and landslides using InSAR mapping
data. Shibata et al. (2020) reported coseismic groundwater-level changes in wells.
This special issue presents the characteristics of the2018 Hokkaido earthquake. It also presents the tectonic
implications as well as clues for seismic hazard assessments. This Mw6.6 earthquake demonstrated that evena moderate-sized earthquake can halt social activity inJapan, which is equipped with advanced earthquake disaster countermeasures. The possible accumulation ofstress on nearby major active faults provides important
information regarding disaster preparedness. Further
investigations of the complex Hidaka collision zone fromgeoscientific and earthquake engineering perspectives
are expected in future work, which will improve existingdisaster countermeasures.

Abbreviations
JMA: Japan Metrological Agency; GNSS: Global Navigation Satellite System;
ETAS: Epidemic type aftershock sequences; PGA: Peak ground acceleration;
PGV: Peak ground velocity; Qs: S-wave attenuation; Qp: P-wave attenuation;
SAR: Synthetic aperture radar; InSAR: Interferometric SAR.
Acknowledgements
We are very grateful to the authors who submitted papers to the special issue
and to the referees, who provided valuable suggestions and recommendations. We are also thankful to the Editor-in-Chief, Dr. Ogawa and Dr. Furuya
who recommended for the organization of this special issue.
Authors’ contributions
HT, NT, MC, GM, SK and TY served as guest editors for this special issue. All
authors read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.
Author details
1
Institute of Seismology and Volcanology, Faculty of Science, Hokkaido University, Sapporo, Japan. 2 Faculty of Engineering, Hokkaido University, Sapporo,
Japan. 3 Disaster Prevention Research Institute, Kyoto University, Uji, Japan.
4
Institute of Earthquake Forecasting, China Earthquake Administration, Beijing, China. 5 Building Research Institute, Tsukuba, Japan. 6 Faculty of Science,
Ibaraki University, Mito, Japan.
Received: 10 April 2020 Accepted: 14 April 2020

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Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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The 2018 Hokkaido Eastern Iburi Earthquake and Hidaka arc-arc collision system

FRONTIER LETTER

Open Access

Structural heterogeneity in and around the fold-and-thrust belt of the
Hidaka Collision zone, Hokkaido, Japan and its relationship to the aftershock
activity of the 2018 Hokkaido Eastern Iburi Earthquake
Takaya Iwasaki*, Noriko Tsumura, Tanio Ito, Kazunori Arita, Makoto Matsubara, Hiroshi Sato, Eiji Kurashimo,
Naoshi Hirata, Susumu Abe, Katsuya Noda, Akira Fujiwara, Shinsuke Kikuchi and Kazuko Suzuki
Earth, Planets and Space 2019, 71, 103 DOI: 10.1186/s40623-019-1081-z
Received: 10 March 2019, Accepted: 11 September 2019, Published: 27 September 2019

Abstract
The Hokkaido Eastern Iburi Earthquake (M = 6.7) occurred on Sep. 6, 2018 in the southern part of Central Hokkaido, Japan. Since Paleogene, this region has
experienced the dextral oblique transpression between the Eurasia and North American (Okhotsk) Plates and the subsequent collision between the
Northeast Japan Arc and the Kuril Arc due to the oblique subduction of the Pacific Plate. This earthquake occurred beneath the foreland fold-and-thrust beltof the Hidaka Collision zone developed by the collision process, and is characterized by its deep focal depth (~ 37 km) and complicated rupture process. The
reanalyses of controlled source seismic data collected in the 1998–2000 Hokkaido Transect Project revealed the detailed structure beneath the fold-andthrust belt, and its relationship with the aftershock activity of this earthquake. Our reflection processing using the CRS/MDRS stacking method imaged forthe first time the lower crust and uppermost mantle structures of the Northeast Japan Arc underthrust beneath a thick (~ 5–10 km) sedimentary package ofthe fold-and-thrust belt. Based on the analysis of the refraction/wide-angle reflection data,
the total thickness of this Northeast Japan Arc crust is only 16–22 km. The Moho is at depthsof 26–28 km in the source region of the Hokkaido Eastern Iburi Earthquake. Our hypocenter
determination using a 3D structure model shows that most of the aftershocks are distributed
in a depth range of 7–45 km with steep geometry facing to the east. The seismic activity isquite low within the thick sediments of the fold–thrust belt, from which we find no
indication on the relationship of this event with the shallow (< 10–15 km) and rather flat
active faults developed in the fold-and-thrust belt. On the other hand, a number ofaftershocks are distributed below the Moho. This high activity may be caused by the cold
crust delaminated from the Kuril Arc side by the arc–arc collision, which prevents the thermal
circulation and cools the forearc uppermost mantle to generate an environment morefavorable for brittle fracture.

Keywords: The Hokkaido Eastern Iburi Earthquake, Hokkaido, Arc–arc collision,

Fold-and-thrust belt, Seismic structure, Controlled source seismic experiments

Graphical abstract*Corresponding author: Takaya Iwasaki, iwasaki@eri.u-tokyo.ac.jp

FULL PAPER

Open AccessPeak ground motions and characteristics of nonlinear site
response during the 2018 Mw 6.6 Hokkaido eastern Iburi
earthquake
Yadab P. Dhakal*, Takashi Kunugi, Takeshi Kimura, Wataru Suzuki and Shin Aoi
Earth, Planets and Space 2019, 71, 56 DOI: 10.1186/s40623-019-1038-2
Received: 21 January 2019, Accepted: 13 May 2019, Published: 22 May 2019

Abstract
The observed peak ground accelerations and peak ground velocities (PGVs) of the 2018 Mw 6.6 Hokkaido eastern Iburi earthquake generally
followed the median values from ground motion prediction equations with reasonable errors at fault distances ≥ 50 km. However, at smaller
distances, the equations significantly underestimated the peak ground motions, and it was eminent for PGVs. A comparison of surface-toborehole spectral ratios of S-waves during the mainshock and other events revealed that the sites at smaller distances experienced various
degrees of nonlinear site response. The two most widely known characteristics of nonlinear site response are the weakening of higherfrequency components and shifting of predominant frequencies to lower ones in comparison with the linear site response. At one of the sitesthat recorded the largest intensity of 7 in JMA scale of 0–7, the latter nonlinear effect was so dominant that the ground motions around thenew predominant frequency got intensified by one order of magnitude in comparison with that during the weak-motions. Two sites, whichwere closely located, recorded vertical peak ground accelerations exceeding 1 g for the up-going motions. The recordings showedasymmetric waveforms and amplitudes characteristics of the nonlinear site response in extreme vertical ground motions recorded during a
few earthquakes in the past. Few sites having lower vertical peak ground accelerations were also suspected of being experienced nonlinear
site response on vertical motions. These findings suggest taking a cautious approach to enumerate
the reduction in amplification at higher frequencies using the single-station horizontal-to-vertical(H-to-V) spectral ratio technique. However, we found that the H-to-V technique was still useful todetect nonlinearity. Finally, an ad hoc equation was derived to correct the nonlinear site
amplification in predicting horizontal PGVs with respect to one of the most widely used attenuation
models in Japan. The results indicated that the effect was much stronger for a larger input motion
than that for a proportional change in the Vs30 values.

Keywords: Nonlinear site response, Degree of nonlinearity, Ground motion prediction equations,Spectral ratios, Peak ground acceleration, Peak ground velocity, The 2018 Hokkaido
eastern Iburi earthquake*Corresponding author: Yadab P. Dhakal, ydhakal@bosai.go.jp
4Graphical abstract

Earth, Planets and Space

FULL PAPER

Open Access

Seismic potential around the 2018 Hokkaido Eastern Iburi
earthquake assessed considering the viscoelastic
relaxation
Makiko Ohtani* and Kazutoshi Imanishi
Earth, Planets and Space 2019, 71, 57 DOI: 10.1186/s40623-019-1036-4
Received: 29 January 2019, Accepted: 13 May 2019, Published: 22 May 2019

Abstract
The 2018 Mj 6.7 Hokkaido Eastern Iburi earthquake (Iburi earthquake) occurred near the eastern boundary fault zone of the Ishikari lowlands,
which is composed of a northern and southern fault. Aftershock distribution suggests the existence of a previously unknown fault (theShallow Iburi (SI) fault) at a shallower extension of the Iburi earthquake fault. In the present study, we examined the seismic potential of thenorthern, southern, and SI faults based on seismological analysis and numerical simulations. The aftershock focal mechanisms infer thepresent-day stress field that is characterized by an ENE–SWS compression around the target faults. Slip tendency analysis shows that all target
faults originally have high slip potential under the estimated stress field. We, therefore, evaluated earthquake occurrence potential on thetarget faults influenced by the Iburi earthquake based on the Coulomb stress (ΔCFF). We consider postseismic viscoelastic deformation in theviscoelastic medium with a three-dimensional structure. The present paper shows one possible scenario based on a model incorporating
information currently available. The most of the entire SI fault was brought closer to rupture justafter the earthquake, indicated by the positive ΔCFF, which is consistent with the activation ofseismicity in this region. The ΔCFF continues to increase over many years after the earthquake,
which may imply a growing risk of seismic hazards along the SI fault. The distribution of the ΔCFF
along the southern fault and the southern half of the northern fault is characterized by a similar
depth-dependent pattern. The faults were brought closer to rupture just after the earthquake,
indicated by the positive ΔCFF, except for mid-depths along the faults. Then, the ΔCFF increases atshallow depths for a few decades after the earthquake, which suggests a continuous build-up ofstress. Although the ΔCFF decreases at the deep depths for a few decades after the earthquake, it isinsufficient to return the stress level to that before the earthquake. These results suggest that all
target faults are in the state of increasing seismic risk after the Iburi earthquake.

Keywords: 2018 Hokkaido Eastern Iburi earthquake, Seismic potential, Stress field estimation,
Slip tendency analysis, Time-dependent Coulomb stress, Viscoelastic relaxation

Graphical abstract*Corresponding author: Makiko Ohtani, ohtani.m@aist.go.jp

FULL PAPER

Open Access

Rupture process of the 2018 Hokkaido Eastern Iburi
earthquake derived from strong motion and geodetic data
Hiroaki Kobayashi*, Kazuki Koketsu and Hiroe Miyake
Earth, Planets and Space 2019, 71, 63 DOI: 10.1186/s40623-019-1041-7
Received: 28 February 2019, Accepted: 22 May 2019, Published: 31 May 2019

Abstract
The source rupture process of the 2018 Hokkaido Eastern Iburi earthquake was investigated by performing a joint inversion
analysis using the strong motion and geodetic data. A fault model that consists of two fault planes was constructed byconsidering the relocated aftershock distribution and the focal mechanisms. The inversion result showed that the large slip
occurred at approximately 22 km depth, which was much shallower than the hypocentral depth. Our results showed that therupture initiated on the minor fault plane around the hypocenter and the major fault plane started to rupture 4–6 s after the
rupture initiation. Although the shape of the minor fault plane has not been clearly determined, the major fault plane appears
to be high-angle, east-dipping. An additional inversion with only the major fault plane showed that the strong ground motion
near the source was mainly generated from the major fault plane. The total seismic moment estimated by the inversion withthe two fault-plane model was 1.1 × 1019 Nm, which yielded an Mw of 6.6. Using the inversion result of the two fault-planemodel, we simulated the ground surface and borehole
waveforms at strong motion station IBUH03 where thelarge velocity pulse was observed on the ground surface.
The simulations suggest that this large velocity pulse was
generated from the combination of the large slip of thesource and large site amplification of the velocity
structure between the ground surface and borehole
seismometers.

Keywords: Rupture process, 2018 Hokkaido Eastern Iburi
earthquake, Strong motion, Geodetic data

Graphical abstract*Corresponding author: Hiroaki Kobayashi, kohiroak@kobori-takken.co.jp; khiro@eri.u-tokyo.ac.jp
5The 2018 Hokkaido Eastern Iburi Earthquake and Hidaka arc-arc collision system

FULL PAPER

Open Access

Detection and interpretation of local surface deformation from the 2018
Hokkaido Eastern Iburi Earthquake using ALOS-2 SAR data
Satoshi Fujiwara*, Takayuki Nakano, Yu Morishita, Tomokazu Kobayashi, Hiroshi Yarai,
Hiroshi Une and Kyonosuke Hayashi
Earth, Planets and Space 2019, 71, 64 DOI: 10.1186/s40623-019-1046-2
Received: 5 March 2019, Accepted: 26 May 2019, Published: 3 June 2019

Abstract
We identified and analyzed surface displacements associated with the 2018 Hokkaido Eastern Iburi Earthquake in northern Japan using satellite
radar interferograms from the Advanced Land Observing Satellite 2. The data generally show elastic deformation caused by the main earthquake
as well as numerous complex surface displacements that cannot be explained by the motion of the seismic source fault. We identified three
distinct phenomena: linear surface displacements representing secondary earthquake faults west of the epicenter, surface deformation caused
by liquefaction in urban and coastal areas, and coherence changes in the interferograms due to landslides in mountainous areas and liquefaction
in urban areas. The linear surface displacements show reverse fault motion with low
dip angles and appear to be a geographic extension of known active faults; however,
it is unlikely that these displacements were directly connected to the source fault of
the main earthquake. Although there is no evidence that they generated strong
seismic waves at the time of the main earthquake, there is a possibility that they
represent active fault traces and could be the sources of large earthquakes in the
future. Therefore, such linear surface displacements can be used to identify potentially
dangerous hidden active faults. The interferograms reveal that liquefaction in urban
areas occurred in low areas artificially filled during past residential development.
Coherence-change maps drawn from the interferograms were useful for detecting
liquefaction, but their high sensitivity limited their application for landslide detection
in mountainous areas; the phase noise deviation method was more practical for
purposes such as rapid response or mitigation. Our methods have the potential toallow improved mapping of local hazards in other areas and can be applied to urban
planning and/or safety assessments.

Keywords: 2018 Hokkaido Eastern Iburi Earthquake, ALOS-2, SAR interferometry,

Graphical abstract

Surface fault, Landslide, Liquefaction*Corresponding author: Satoshi Fujiwara, fujiwara-s2vq@mlit.go.jp

FULL PAPER

Open AccessCause of destructive strong ground motion within 1–2 s in Mukawa
town during the 2018 Mw 6.6 Hokkaido eastern Iburi earthquake
Nobuo Takai*, Michiko Shigefuji, Jun Horita, Shingo Nomoto, Takahiro Maeda,
Masayoshi Ichiyanagi, Hiroaki Takahashi, Hiroaki Yamanaka, Kosuke Chimoto,
Seiji Tsuno, Masahiro Korenaga and Nobuyuki Yamada
Earth, Planets and Space 2019, 71, 67 DOI: 10.1186/s40623-019-1044-4
Received: 5 March 2019, Accepted: 24 May 2019, Published: 13 June 2019

Abstract

Keywords: The 2018 Hokkaido eastern Iburi earthquake,

Strong ground motion, Velocity structure,
Temporary strong motion observation,
Microtremor measurement, Multichannel
analysis of surface wave, S-wave velocity, Siteamplification

Frequency (Hz)

10

10

1

0
1
−1
0

1

2

Distance (km)Graphical abstract*Corresponding author: Nobuo Takai, tki@eng.hokudai.ac.jp
63

4

H/V spectral ratio

We investigated the cause of destructive ground motion during the 2018 Hokkaido eastern Iburi earthquake. We conducted strong
motion observations of aftershocks and microtremors and the surface wave method in the damaged areas of the town of Mukawa,
Hokkaido prefecture, Japan. The ground accelerations were continuously recorded during a period of approximately 3 months after
the main shock on September 6, 2018. The heavily damaged buildings were mainly situated around the strong motion station(HKD126) in Mukawa town. Such concentration of damage can be explained by the strong power that was observed in the 1–2 speriod of the response spectrum at this station. We estimated the S-wave velocity profiles of this station site and a temporary stationsite that was installed on a nearby hill. The estimated S-wave velocity, which was inverted from phase velocity structures with themicrotremor array and the surface wave method observations explained the difference in the SH-wave amplification characteristics
between the two sites. An analysis of HKD126 and the temporarily observed records clearly indicates the strong effects of the local
geological conditions on the heavily damaged area of
Mukawa. The strong ground motion power generated during
South
North
the main shock in Mukawa for 1–2 s period was mainly
HKD126
AOB
MKW01
HV10
HV09
HV08
HV07
HV06
HV05
HV04 HV03 HV01 HV02
amplified by this shallow underground velocity structure.
logEarth, Planets and Space

FULL PAPER

Open AccessCoseismic changes in subsurface structure associated withthe 2018 Hokkaido Eastern Iburi Earthquake detectedusing autocorrelation analysis of ambient seismic noise
Hiroki Ikeda and Ryota Takagi*
Earth, Planets and Space 2019, 71, 72 DOI: 10.1186/s40623-019-1051-5
Received: 24 February 2019, Accepted: 18 June 2019, Published: 27 June 2019

Abstract
Autocorrelation analysis using ambient noise is a useful method to detect temporal changes in wave velocity and scattering property.
In this study, we investigated the temporal changes in seismic wave velocity and scattering property in the focal region of the 2018
Hokkaido Eastern Iburi Earthquake. The autocorrelation function (ACF) was calculated by processing with bandpass filters to enhance
1–2 Hz frequency range, with aftershock removal, and applying the one-bit correlation technique. The stretching method was used todetect the seismic wave velocity change. After the mainshock, seismic velocity reductions were observed at many stations. At N.
AMAH and ATSUMA, which are located close to the mainshock, we detected 2–3% decreases in seismic wave velocity. We comparedparameters indicating strong ground motion and showed thepossibility of correlations with peak dynamic strain and seismic
velocity reduction. We also investigated the relationship between
waveform correlation and lag time, using ACFs from before and after
the mainshock, and detected distortion of the ACF waveform. The
source of the waveform decorrelation was estimated to be locatednear the maximum coseismic slip, at around 30 km depth. Thus,
distortion of the ACF waveform may reflect the formation of cracks,
due to faulting at approximately 30 km depth.

Keywords: Seismic velocity changes, Ambient noise, Scatterer
distribution change, Autocorrelation function,
Hokkaido Eastern Iburi Earthquake, Seismic
interferometry

Graphical abstract*Corresponding author: Ryota Takagi, ryota.takagi.c1@tohoku.ac.jp

FULL PAPER

Open Access

Topographic effects on frequency-size distribution oflandslides triggered by the Hokkaido Eastern Iburi
Earthquake in 2018
Mio Kasai* and Takashi Yamada
Earth, Planets and Space 2019, 71, 89 DOI: 10.1186/s40623-019-1069-8
Received: 18 February 2019, Accepted: 6 August 2019, Published: 17 August 2019

Abstract
This study examined the frequency-size distribution of 6117 landslides spread over 440 km2 in Iburi Subprefecture, Hokkaido, Japan, inducedby the Hokkaido Eastern Iburi Earthquake (Mw 6.6) on September 6, 2018. The study area is characterized by gently undulating terrain that isfinely dissected by shallow streams and covered predominantly by layers of volcanic products with high water content. Most of the landslides
were shallow landslides, and their slip surfaces often formed in a layer of volcanic soil called the “Ta-d,”
deposited at 9000 ybp. Low ridges separating small catchments allowed individual landslides to coalesce
in many locations. The average size of landslides was 7160 m2. Landslide size tended to increase with slope
angle up to 20° to 25° and then decrease with further increase of slope angle. About half of the landslides
occurred in a feature with both concave planform and profile curvature, and their average size was 8720
m2. In contrast, 17% of the total landslides occurred in the case of both curvatures being convex, and theiraverage size was 5190 m2. The results indicated that the accumulation of saturated soil in concave features
provided more opportunities for landslides of large sizes. The frequency-size distribution of the landslides
presented high rollover, 5.0 × 10−3 km2, but the exponent of power law decay for medium to large
landslides, − 2.46, was not largely different from those of studies in other locations. Compared with otherseismically caused examples, the landslides triggered by the Hokkaido Eastern Iburi Earthquake can becharacterized as more clustered, more numerous, and larger in size for the moment magnitude of theearthquake. Conversely, the magnitude scale for the landslide event estimated from the total landslidearea was equivalent to that of a region struck by an earthquake of Mw = 7.0 to 7.4. This study demonstrated
that gently undulating regions can produce unexpectedly large and frequent landslides when struck by anintense earthquake, and when soil layers vulnerable to ground shaking cover the ground.

Keywords: Frequency-size distribution, Hokkaido Eastern Iburi Earthquake, Shallow landslides, Slope
angle, Slope curvature, Volcanic soil

*Corresponding author: Mio Kasai, kasaim@for.agr.hokudai.ac.jp
7Graphical abstract

The 2018 Hokkaido Eastern Iburi Earthquake and Hidaka arc-arc collision system

FULL PAPER

Open Access

Source rupture process of the 2018 Hokkaido Eastern Iburi earthquake
deduced from strong-motion data considering seismic
wave propagation in three-dimensional velocity structure
Kimiyuki Asano* and Tomotaka Iwata
Earth, Planets and Space 2019, 71, 101 DOI: 10.1186/s40623-019-1080-0
Received: 1 May 2019, Accepted: 3 September 2019, Published: 18 September 2019

Abstract
The source rupture process of the 2018 Hokkaido Eastern Iburi earthquake (MJMA 6.7) was analyzed by a kinematic waveform inversion method
using strong-motion data in 0.04–0.5 Hz. This earthquake occurred close to the Hidaka Collision Zone and the Ishikari depression, where thecrustal structure is rather complex. Thus, we used a three-dimensional velocity structure model to compute the theoretical Green’s functions by
the finite difference method. A source fault model with strike-angle variation was set based on the spatial distribution of the early aftershocks.
The strong-motion stations used for the source inversion were selected based on the result of forward ground motion simulation of a moderate
aftershock. The slip in the first 5 s was relatively small, but an area of significant slip with peak slip of 1.7 m was found in the depth range from 22
to 32 km. The rupture propagated upward mainly in the southwest direction. Based on the regional crustal structure and the configuration of the
Moho discontinuity, the large-slip area was thought to be located in the lower crust, and its rupture did not reach the upper part of thecontinental crust. Most of the early aftershocks occurred around the large-slip area. The later aftershocks at the depth shallower than 20 km
occurred outside the causative source fault of the mainshock. Threedimensional ground motion simulation demonstrated that theheterogeneous source process and the three-dimensional basin and crustal
velocity structure brought a large velocity pulse to an area to the southwestof the source fault, where the largest PGV was observed during themainshock. The spatial distribution of the simulated PGV resembled the
The intrusion structure of high velocity materials
observed PGV distribution except some sites located inside the Ishikari
depression where thick Quaternary soft low-velocity sediments exist at the0
top of the basin.
Vp (g)
6.0
7.0

process, Kinematic source inversion, Strong-motion data,
Three-dimensional velocity structure, Ground motion simulation

60
Graphical
abstract2018 M6.7

2018 M6

8.0 materials
The intrusion structure of high velocity

0

6.5

7.2M6.7
2018

20
40

6.0

7.
8

2.7

7.2

7.2

7.
8

40
40 60
7
60 A
E .2
7 C

7.2

7.8

7.2 8.4
7.8 8.4

Vs

2.7

3.9

0

1970
540.20

00

E 5
A
C
-40

0

44˚
Vp (k)(i)Y−60km44˚
xA

3.6
Vp[km/s] Vp[km/s]

6.6 6.6

7.50
8.35 7.98
8.50 8.35 8.50
6.086.85 7.13
6.85
7.137.98
7.50

40
40

Y−50kmC

6

Depth_km

(c)
(e)

80
-40

4.5

3.9
3.9

3.9

Y−40km
Y−60km
7.8 EY10km

3.

20
20
6.08

8.4

Y−60km0 40
40-40 -40 0
80
Distance_km
0 5.4 Distance_km

3.9

3.31982
Vp (h)
(j)
3.
(l)
6

(l) 3.3
Vp 6.61970

60 E .2

4.2

4.5
3.9

5

6.6

20 40

6.0

7.2

(f)

(b)
(f)

D

Y−50km1982

4.2

Depth_km
Depth_km

0

X
B
D

Y30km
Y−30kmY−50km

2018Vs
M6

4.2
4.2

4.

7.2

D

.6

8.0
8.5

2.7

Depth_km
Depth_km
Depth_km

Depth_km

2
7.

7.2

D

0 20
20

Depth_km

2018 M6.7Vp (k)3

6.6

4.5 Y−40km

3.6
3.9

6

40
(e)
60
40

60
80
60 X
B

3.9

42˚

B

4.2
B 140˚
D

142˚

D 42˚
E
Graphical abstract Vs[km/s] Vs[km/s]
3.9
4.2
2

3.70 3.93 4.15 4.38 4.60 4.83 4.90

Depth_km

2
7.

D

0

8

1970

Vp (g)1970
Vp (k)
(i)
3.

0 5.4

60b

Y−30km

4.2
Vs

6.5

C

7.8 Y−40km

7.
8Depth_km

6.0

7.2
(c)
(e)
7.0
7.0

60
3.937. 4.15 4.38 4.60 4.83 4.90
603.70B
Y−30kmCorresponding author: Saeko Kita, kita@kenken.go.jp

3.9
3.9

5

Depth_km

Depth_km
Depth_km

7.
8

6.6B

4.2

4.2

Depth_km
Depth_km

Y−30km7.2

2040
20 7.5 6.6 6.6

40

Vs

1982
Vp (h)
(j)
3.
6

6.0

1970

200 5.4

60 C

20

4.5

2.7

3.9

40
40
1970
0The intrusion
1982 structure of high velocity(j)1982(b)
Vp materials
3.
7.2
60
6
8.4
20a 606.6A
CAC
Y−40km
3.9
7.8 Y10km
0
40

4.2

Y10km
2018
M6

6

Depth_km

3.

6.6

2

Keywords: Relocations of hypocenters, The Hidaka collision zone, Hokkaido, Seismic structures

B

.2

Y30km
XB 4.2
Y−30kmVp (i)

6.6

7.

I relocated the hypocenters of the 2018 M6.7 Hokkaido Eastern Iburi earthquake and its surroundingarea, using a three-dimensional seismic structure, the double-difference relocation method, and the
JMA earthquake catalog. After relocation, the focal depth of the mainshock became 35.4 km. As
previous studies show, in south-central Hokkaido, the Hidaka collision zone is formed, andanomalous deep and thickened forearc crust material is subducting at depths of less than 70 km.
The mainshock and its aftershocks are located at depths of approximately 10 to 40 km within thelower crust of the anomalous deep and thickened curst near the uppermost mantle material
intrusions in the northwestern edge of this Hidaka collision zone. Like the two previous large events,
the aftershocks of this event incline steeply eastward and appear to be distributed in the deeper
extension of the Ishikari-teichi-toen fault zone. The highly inclined fault in the present study is
consistent with a fault model by a geodetic analysis with InSAR. The aftershocks at depths of 10 to 20
km are located at the western edge of the high-attenuation (low-Qp) zone. These kinds ofrelationships between hypocenters and materials are the same as the 1970 and 1982 events in the
Hidaka collision zone. The anomalous large focal depths of these large events compared with theaverage depth limit of inland earthquakes in Japan could be caused by the locally lower temperature
in south-central Hokkaido. This event is one of the approximately M7 large inland earthquakes that
occurred repeatedly at a recurrence interval of approximately 40 years and is important in thecollision process in the Hidaka collision zone.

3.9
4

A

(b) 1982

0

40

8.0
8.5

2

(c) XB

60 20

Abstract

8.4

Vs

4.5

6

60

0 80

20

Y10km
2018 M6.7

40
A
60

7.

Earth, Planets and Space 2019, 71, 122 DOI: 10.1186/s40623-019-1100-0
Received: 1 May 2019, Accepted: 27 October 2019, Published: 14 November 2019

7.0

3.

Saeko Kita

6.6

Vp
(i)
3.9

6.5

6.6

4.2
Y30km

4.75

Vp (h) Vp (g)

6.0

4.2

7.0 7.2
7.5

40

4.5

A

Y10km

6.0

20
60

8.0

4.0

Open6.6Access2018 M6.7
3.9

intrusion structure of highY30km
velocity materials
80The
X
X
8.5
8.4

A
Characteristics of relocated hypocenters of the 2018600M6.7
Hokkaido Eastern Iburi earthquake and its aftershocks
200(c)with a three-dimensional seismic velocity structure 4020

Vs

Vp (h)

4.25

60

7.2

7.0

Depth_km

7.0
40 7.5

4.2

Depth_km

Vp (g)

6.0

20

4.5

X

Y30km

8.5

2.7

80 X

*Corresponding author: Kimiyuki Asano, k-asano@sms.dpri.kyoto-u.ac.jp

FULL PAPER

6.5

7.0

40 7.54.

Depth_km

20

Keywords: The 2018 Hokkaido Eastern Iburi earthquake, Source rupture20

Y−50km(b)(f) 1982

3.31982
Vp (j)
(l)
3.
6

6.6

3.9

40
60

7.2

144˚

140˚

7.2

1970
7.8

8.4

3.9

1970
500

Earth, Planets and Space

FULL PAPER

Open Access

Characteristics of seismic activity before and after the 2018
M6.7 Hokkaido Eastern Iburi earthquake
Takao Kumazawa*, Yosihiko Ogata and Hiroshi Tsuruoka
Earth, Planets and Space 2019, 71, 130 DOI: 10.1186/s40623-019-1102-y
Received: 25 April 2019, Accepted: 29 October 2019, Published: 29 November 2019

Abstract

1e−02

1e+00

1e+02

We applied the epidemic type aftershock sequence (ETAS) model, the two-stage ETAS model and the non-stationary ETAS model toinvestigate the detailed features of the series of earthquake occurrences before and after the M6.7 Hokkaido Eastern Iburi earthquakeon 6 September 2018, based on earthquake data from October 1997. First, after the 2003 M8.0 Tokachi-Oki earthquake, seismic
activity in the Eastern Iburi region reduced relative to the ETAS model. During this period, the depth ranges of the seismicity were
migrating towards shallow depths, where a swarm cluster, including a M5.1 earthquake, finally occurred in the deepest part of therange. This swarm activity was well described by the non-stationary ETAS model until theM6.7 main shock. The aftershocks of the M6.7 earthquake obeyed the ETAS model until
the M5.8 largest aftershock, except for a period of several days when small, swarm-like
activity was found at the southern end of the aftershock region. However, when we focus
on the medium and larger aftershocks, we observed quiescence relative to the ETAS
model from 8.6 days after the main shock until the M5.8 largest aftershock. For microearthquakes, we further studied the separated aftershock sequences in the naturally
divided aftershock volumes. We found that the temporal changes in the background rate
and triggering coefficient (aftershock productivity) in respective sub-volumes were incontrast with each other. In particular, relative quiescence was seen in the northern deepzones that includes the M5.8 largest aftershock. Furthermore, changes in the b-values of
the whole aftershock activity showed an increasing trend with respect to the logarithmof elapsed time during the entire aftershock period, which is ultimately explained by thespatially different characteristics of the aftershocks.

Keywords: ABIC, AIC, Bayesian method, b-Values, Stationary ETAS model, Two-stage

0

ETAS model, Non-stationary ETAS model

50

100

150

Graphical abstract*Corresponding author: Takao Kumazawa, tkuma@eri.u-tokyo.ac.jp

EXPRESS LETTER

Open Access

Spatiotemporal crustal strain distribution around the
Ishikari-Teichi-Toen fault zone estimated from global
navigation satellite system data
Mako Ohzono*, Hiroaki Takahashi and Chihiro Ito
Earth, Planets and Space 2019, 71, 50 DOI: 10.1186/s40623-019-1024-8
Received: 3 February 2019, Accepted: 6 April 2019, Published: 27 April 2019

Abstract
Based on analyses of global navigation satellite system data since 1996, we investigate the spatiotemporal strain field aroundthe Ishikari-Teichi-Toen fault zone, which is a major active fault zone close to the epicenter of the 2018 Eastern Iburi
earthquake in Hokkaido, Japan. Strain rates during almost whole periods, except for the timings of two distant large interplate
earthquakes and following several years show an E–W to ESE–WNW contraction of ~ 0.1 ppm/year. This strain rate isapproximately an order of magnitude larger than that of the surrounding area. Strain rate disturbances due to large
earthquakes diminish within several years and return to the original level, suggesting that there is a uniform strain
accumulation along this fault zone. Strain rate profiles
that traverse the fault zone are characterized by a major
contraction, corresponding to the Ishikari lowlands
where a significantly thick low seismic velocity layer
exists. A relatively high strain rate around this fault zone
may reflect some amount of inelastic strain
accumulation in addition to the elastic strain
accumulation along the faults originating from complex
fault and crustal structures.

Keywords: 2018 Eastern Iburi earthquake, Ishikari

lowlands, GNSS, Strain concentration zone,
Active faults

Graphical abstract*Corresponding author: Mako Ohzono, m.ohzono@sci.hokudai.ac.jp
9The 2018 Hokkaido Eastern Iburi Earthquake and Hidaka arc-arc collision system

EXPRESS LETTER

Open Access

The 2018 Hokkaido Eastern Iburi earthquake (MJMA = 6.7) was triggered bya strike-slip faulting in a stepover segment: insights from the aftershock
distribution and the focal mechanism solution of the main shock
Kei Katsumata*, Masayoshi Ichiyanagi, Mako Ohzono, Hiroshi Aoyama, Ryo Tanaka, Masamitsu Takada,Teruhiro Yamaguchi, Kazumi Okada, Hiroaki Takahashi, Shin’ichi Sakai, Satoshi Matsumoto,
Tomomi Okada, Toru Matsuzawa, Shuichiro Hirano, Toshiko Terakawa, Shinichiro Horikawa,
Masahiro Kosuga, Hiroshi Katao, Yoshihisa Iio, Airi Nagaoka, Noriko Tsumura, Tomotake Ueno and
the Group for the Aftershock Observations of the 2018 Hokkaido Eastern Iburi Earthquake
Earth, Planets and Space 2019, 71, 53 DOI: 10.1186/s40623-019-1032-8
Received: 24 December 2018, Accepted: 23 April 2019, Published: 20 May 2019

Abstract
The Hokkaido Eastern Iburi earthquake (MJMA = 6.7) occurred on September 6, 2018, in the Hokkaido corner region where the Kurile andnortheastern Japan island arcs meet. We relocated aftershocks of this intraplate earthquake immediately after the main shock by using data froma permanent local seismic network and found that aftershock depths were concentrated from 20 to 40 km, which is extraordinarily deep
compared with other shallow intraplate earthquakes in the inland area of Honshu and Kyushu, Japan. Further, we found that the aftershock area
consists of three segments. The first segment is located in the northern part of the aftershock area, the second segment lies in the southern part,and the third segment forms a stepover between the other two segments. The
hypocenter of the main shock, from which the rupture initiated, is located on the
stepover segment. The centroid moment tensor solution for the main shock
indicates a reverse faulting, whereas the focal mechanism solution determined byusing the first-motion polarity of the P wave indicates strike-slip faulting. To explain
this discrepancy qualitatively, we present a model in which the rupture started as aM
small strike-slip fault in the stepover segment of the aftershock area, followed by
two large reverse faulting ruptures in the northern and southern segments.

Keywords: The Hokkaido Eastern Iburi earthquake, Strike-slip fault, Reverse
fault, Aftershock distribution, Focal mechanism solution, Local
seismic network, Stepover segment

10 km

Graphical abstract*Corresponding author: Kei Katsumata, kkatsu@sci.hokudai.ac.jp

EXPRESS LETTER

Open Access

Geodetically estimated location and geometry of the fault
plane involved in the 2018 Hokkaido Eastern Iburi
earthquake
Tomokazu Kobayashi*, Kyonosuke Hayashi and Hiroshi Yarai
Earth, Planets and Space 2019, 71, 62 DOI: 10.1186/s40623-019-1042-6
Received: 6 March 2019, Accepted: 22 May 2019, Published: 31 May 2019

Abstract
By applying the InSAR method to ALOS-2 SAR data acquired before and after the 2018 Hokkaido Eastern Iburi earthquake, the grounddisplacement fields were successfully mapped. Ground deformation is distributed on the eastern side of an active fault, known as the
Ishikari-Teichi-Toen fault zone (ITTFZ). Uplift of up to ~ 7 cm is distributed throughout the source region, and eastward movement of
up to ~ 4 cm is widely observed on the eastern side of the source region. The fault model constructed by inverting InSAR and GNSS
data under an assumption of a uniform slip on a rectangular fault plane shows reverse fault motion on a plane dipping eastward at
74°. The fault top is positioned around a depth of 15 km, suggesting that the slip significantly occurs deeper than the typicalseismogenic zone for the Japanese island. The fault plane is at a relatively high dip angle, and the shallow extension of the estimatedfault plane does not connect to any known surface traces of the ITTFZ. This may suggest that the fault involved with the 2018 event is
physically separated from the ITTFZ, or that the fault plane
bends to a lower dip angle at a shallow crustal depth. The
fault rupture area is located in a spatially inhomogeneousseismic velocity structure field and the main slip occurs at thewestern edge of a low Vp area which vertically thickens on theeastern side of the fault. This may suggest that the 2018
earthquake occurred at seismic velocity boundary. An
estimate of Coulomb Failure Function change suggests thatthe static stress change due to the 2018 event can promote
reverse fault slip on the southern part of the ITTFZ.

Keywords: 2018 Hokkaido Eastern Iburi earthquake, Crustal
deformation, InSAR, Fault model

Graphical abstract*Corresponding author: Tomokazu Kobayashi, kobayashi-t96dv@mlit.go.jp
10Earth, Planets and Space

EXPRESS LETTER

Open Access

Source parameters and site amplifications estimated bygeneralized inversion technique: focusing on the 2018
Hokkaido Iburi-Tobu earthquake
Kenichi Nakano* and Hiroshi Kawase
Earth, Planets and Space 2019, 71, 66 DOI: 10.1186/s40623-019-1047-1
Received: 24 February 2019, Accepted: 4 June 2019, Published: 13 June 2019

Abstract
The 2018 Hokkaido Iburi-Tobu earthquake occurred on September 6. In this earthquake, seismic intensity 7 was measured at
Shikanuma, Atsuma Town, and severe damage occurred in Hokkaido. The strong motions were recorded at observation stations
including those nearby the epicenter, and the seismograms at a few stations close to the epicenter showed pulse-like waveforms. In
the region called Yufutsu Plain, the underground structure is quite complex because Yufutsu Plain is located at the western margin of
the Hidaka collision zone. Thus, the mechanism of generating the observed high-amplitude ground motions in this area has not beenclarified yet. We performed the generalized inversion technique to estimate the fundamental characteristics of source terms and site
amplifications at Yufutsu area to investigate the mechanism mentioned above. We foundthat the stress drop of the mainshock of the 2018 Hokkaido Iburi-Tobu earthquake wasapproximately 10Mpa, which corresponds to the upper limit of the crustal earthquakes in
the past, and that the short-period level A was approximately 1.17 × 1019 N·m. We also
evaluated the site amplification at sites close to the aftershock zone. We found that the sites
in the north with relatively high altitudes showed dominant frequencies above several Hz,
while the sites in the south with relatively low altitudes showed dominant frequencies ofapproximately 1 Hz or lower. We also found that the site amplifications were quite large atthree sites (BBC, HKD126 and IBUH03) where we observed clear pulse-like waveforms, andconfirmed that they all showed dominant frequencies at approximately 1 Hz.

Keywords: 2018 Hokkaido Iburi-Tobu earthquake, Stress drop, Short-period level A, Siteamplification

Graphical abstract*Corresponding author: Kenichi Nakano, nakano.kenichi@ad-hzm.co.jp

EXPRESS LETTER

Open Access

Estimation of convergence boundary location and velocity
between tectonic plates in northern Hokkaido inferred by
GNSS velocity data
Chihiro Ito*, Hiroaki Takahashi and Mako Ohzono
Earth, Planets and Space 2019, 71, 86 DOI: 10.1186/s40623-019-1065-z
Received: 26 February 2019, Accepted: 24 July 2019, Published: 2 August 2019

Abstracta Boundary locationb Fitting results
North area
15

velocity[mm/yr]

0848
0001
0101

0779

(1)

45˚

(2)

0849

0778

0501

0103

0850
0851

0104

0782

0855

0858

0110
0506

0862

0008
0509

0863
0510

0508

0865

0785
0872 2005

15

0504

0857
0860

0854

0107

0783

(3)

0007

0867

0871

43˚

0013

884

0877

01

0

0521

15

0517
0880

0882

0518 0879

0014
0522

0129
0135

1150
0526

0792
0018

0789

0012
0885

0887

0523
137

0790

0132

0133

0134

0136
0890

0139

(4)

0793

0141
0016
0891
0144

deformation, Hokkaido

Whole area

0520
0128

0142

Keywords: GNSS measurements, Collision zone, Amur plate, Crustal

5

0874

0514
0876

10

0787

0117

0516

South area

0002

0003

44˚

5

0

0502

0853
0106

10

0780

0852 0105
0781

velocity[mm/yr]

0102

42˚ 47100 kmvelocity[mm/yr]

The present location of the tectonic boundary and the convergence rate
between the Amur and Okhotsk plates in northern Hokkaido, Japan, were
herein estimated from the velocity field using data from a continuous GNSS
network. The observed velocity profiles are in agreement with the theoreticalones calculated from a tectonic block collision model. The estimatedkinematic boundary agrees with both geological and seismic boundaries.
Overall, this indicates that the geological boundary acts like a mechanicalone. The calculated convergence velocity of 14.0–16.5 mm/year is consistent
with predictions from regional plate motion models and suggests that a
considerable amount of interplate convergence is in progress along thisboundary. Deep crustal seismicity is also in agreement with the estimatedelastic thickness of 20.5–25.5 km. The non-occurrence of large earthquakes
during the past several centuries, and the estimated convergence velocity
suggest that there is a high potential for a large event in the near future.

10

5

0019

0
141˚

142˚

143˚

−150

−100

−50

0

50

distance[km]Graphical abstract*Corresponding author: Chihiro Ito, chr-0521-i@eis.hokudai.ac.jp
11

100

150

The 2018 Hokkaido Eastern Iburi Earthquake and Hidaka arc-arc collision system

EXPRESS LETTER

Open AccessElectrical resistivity modeling around the Hidaka collisionzone, northern Japan: regional structural background of
the 2018 Hokkaido Eastern Iburi earthquake (Mw 6.6)
Hiroshi Ichihara*, Toru Mogi, Hideyuki Satoh and Yusuke Yamaya
Earth, Planets and Space 2019, 71, 100 DOI: 10.1186/s40623-019-1078-7
Received: 5 March 2019, Accepted: 26 August 2019, Published: 18 September 2019

Abstract
The Hidaka collision zone, the collision boundary between the NE Japan and Kurile arcs, is known to be an ideal region to
study the evolution of island arcs. The hypocenter of the 2018 Hokkaido Eastern Iburi earthquake (Mw 6.6) in the western partof the Hidaka collision zone was unusually deep for an inland earthquake, and the reverse fault that caused the earthquake
has an uncharacteristically steep dip. In this study, we used three-dimensional inversion to reanalyze broadband
magnetotelluric data acquired in the collision zone. The inverted resistivity model showed a significant area of high resistivity
around the center of the collision boundary. We also identified a conductive zone beneath an area of serpentinite mélange ina zone of high P–T metamorphic rocks west of the high-resistivity zone. The conductive zone possibly reflects areas rich inpore fluids related to the formation and elevation of theserpentinites. Sensitivity tests indicated the need for
additional magnetotelluric survey data to delineate theresistivity distribution around the epicentral area of the2018 earthquake although the resistivity model showeda conductive zone in this area.

Keywords: Hidaka collision zone, Magnetotellurics,

Electrical conductivity, Resistivity,
Serpentinite, 2018 Hokkaido Eastern Iburi
earthquake

Graphical abstract*Corresponding author: Hiroshi Ichihara, h-ichi@seis.nagoya-u.ac.jp

EXPRESS LETTER

Open AccessThree-dimensional S-wave attenuation structure in andaround source area of the 2018 Hokkaido Eastern Iburi
Earthquake, Japan
Ryoichi Nakamura* and Takahiro Shiina
Earth, Planets and Space 2019, 71, 114 DOI: 10.1186/s40623-019-1095-6
Received: 8 May 2019, Accepted: 23 October 2019, Published: 6 November 2019

Abstract
We investigate S-wave attenuation (Qs) structure in and around Hokkaido, Japan, including the source area of the 2018 Hokkaido Eastern Iburi
Earthquake (M 6.7) and its aftershocks. From the strong-motion seismograms recorded in the nationwide seismograph network in Japan, we
compute spectrum amplitude at 1–10 Hz with every 1 Hz and then estimate Qs structure by adopting tomographic inversion technique. The
obtained Qs structure suggests that lateral variations of Qs are remarkably formed in the study areas. The low-Qs anomalies distributing northto south are imaged in both sides of the Hidaka mountain range which is built in the central Hokkaido. These characteristics structures of Qs
are extended from the ground surface to depths by about 50 km, indicating that heterogeneous structures resulted from the collision of theKuril and northeastern Japan arcs develop at the depth ranges. Aroundthe source area of the 2018 Hokkaido Eastern Iburi Earthquake, theabrupt changes of Qs values are identified. The 2018 Hokkaido Eastern
Iburi Earthquake and its aftershocks seem to be arraigned on thehigh-Qs zones, whereas the low-Qs anomalies are also imaged next tothe high-Qs zones. The boundary of the low- and high-Qs zones lies
with near vertical, corresponding to aftershock distributions of the2018 Hokkaido Eastern Iburi Earthquake. This agreement would
propose that the heterogeneities formed due to the arc–arc collision
would characterize faulting processes in the main shock of the
Hokkaido Eastern Iburi Earthquake and the activities of its aftershocks.
1

Keywords: The 2018 Hokkaido Eastern Iburi Earthquake, S-wave

attenuation, The arc–arc collision zone, Tomography

Fig. 1*Corresponding author: Ryoichi Nakamura, naka@eri.u-tokyo.ac.jp
12Earth, Planets and Space

EXPRESS LETTER

Open Access

Estimation of the source process and forward simulation of
long-period ground motion of the 2018 Hokkaido Eastern
Iburi, Japan, earthquake
Hisahiko Kubo*, Asako Iwaki, Wataru Suzuki, Shin Aoi and Haruko Sekiguchi
Earth, Planets and Space 2020, 72, 20 DOI: 10.1186/s40623-020-1146-z
Received: 24 December 2019, Accepted: 30 January 2020, Published: 18 February 2020

Abstract
In this study, we investigate the source rupture process of the 2018 Hokkaido Eastern Iburi earthquake in Japan (MJMA 6.7) andhow the ground motion can be reproduced using available source and velocity models. First, we conduct a multiple-timewindow kinematic waveform inversion using strong motion waveforms, which indicates that a large slip area located at a
depth of 25–30 km in the up-dip direction from the hypocenter was caused by a rupture propagating upward 6–12 s after its
initiation. Moreover, the high-seismicity area ofaftershocks did not overlap with the large-sliparea. Subsequently, using the obtained source
model and a three-dimensional velocity structure
model, we conduct a forward long-period (< 0.5Hz) ground-motion simulation. The simulation wasable to reproduce the overall ground-motion
characteristics in the sedimentary layers of the
Ishikari Lowland.

Keywords: The 2018 Hokkaido Eastern Iburi

earthquake, Source process, Forward
simulation, Long-period groundmotionGraphical abstract*Corresponding author: Hisahiko Kubo, hkubo@bosai.go.jp

EXPRESS LETTER

Open AccessCoseismic changes in groundwater level during the 2018
Hokkaido Eastern Iburi earthquake
Tomo Shibata*, Ryo Takahashi, Hiroaki Takahashi, Takanori Kagoshima, Naoto Takahata, Yuji Sano andDaniele L. Pinti
Earth, Planets and Space 2020, 72, 23 DOI: 10.1186/s40623-020-01152-y
Received: 24 May 2019, Accepted: 17 February 2020, Published: 26 February 2020

Abstract
Six wells were continuously monitored in Hokkaido, northern Japan, to relate groundwater level changes to regional seismic
activity. Groundwater level changes following the 2018 Hokkaido Eastern Iburi earthquake were detected in three of the six
wells, even though they are located hundreds of kilometers from the epicenter. The groundwater level changes are
qualitatively consistent with the volumetric strain induced by the earthquake. We analyzed groundwater level responses tothe M2 tidal constituent before and after the earthquake, but
related changes in amplitude and phase shifts remained
within the usual variation. Observed coseismic change was
explained by the response to the M2 tidal constituent
component and the calculated volumetric strain for one of
the wells, where groundwater level decreased. The observedchange in the other two was found to be much greater thanthe corresponding estimates of the volumetric strain.

Keywords: Groundwater level, Coseismic change, 2018
Hokkaido Eastern Iburi earthquake, Tidal
response

Graphical abstract*Corresponding author: Tomo Shibata, swn05‑toshibat@bep.vgs.kyoto‑u.ac.jp
13

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Contents
Special issue “The 2018 Hokkaido Eastern Iburi Earthquake and Hidaka arc–arc collision system”. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hiroaki Takahashi, Nobuo Takai, Masahiro Chigira, Guojie Meng, Saeko Kita and Takuji Yamada
Structural heterogeneity in and around the fold-and-thrust belt of the Hidaka Collision zone, Hokkaido, Japan and its relationshipto the aftershock activity of the 2018 Hokkaido Eastern Iburi Earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . Takaya Iwasaki, Noriko Tsumura, Tanio Ito, Kazunori Arita, Makoto Matsubara, Hiroshi Sato, Eiji Kurashimo,
Naoshi Hirata, Susumu Abe, Katsuya Noda, Akira Fujiwara, Shinsuke Kikuchi and Kazuko Suzuki

Peak ground motions and characteristics of nonlinear site response during the 2018 Mw 6.6 Hokkaido eastern Iburi
earthquake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yadab P. Dhakal, Takashi Kunugi, Takeshi Kimura, Wataru Suzuki and Shin Aoi
Seismic potential around the 2018 Hokkaido Eastern Iburi earthquake assessed considering the viscoelastic relaxation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Makiko Ohtani and Kazutoshi Imanishi
Rupture process of the 2018 Hokkaido Eastern Iburi earthquake derived from strong motion and geodetic data
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hiroaki Kobayashi, Kazuki Koketsu and Hiroe Miyake
Detection and interpretation of local surface deformation from the 2018 Hokkaido Eastern Iburi Earthquake using ALOS-2
SAR data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Satoshi Fujiwara, Takayuki Nakano, Yu Morishita, Tomokazu Kobayashi,
Hiroshi Yarai, Hiroshi Une and Kyonosuke Hayashi

Cause of destructive strong ground motion within 1–2 s in Mukawa town during the 2018 Mw 6.6 Hokkaido eastern
Iburi earthquake . . . . . . . . . . . . . Nobuo Takai, Michiko Shigefuji, Jun Horita, Shingo Nomoto, Takahiro Maeda, Masayoshi Ichiyanagi,

Hiroaki Takahashi, Hiroaki Yamanaka, Kosuke Chimoto, Seiji Tsuno, Masahiro Korenaga and Nobuyuki Yamada

Coseismic changes in subsurface structure associated with the 2018 Hokkaido Eastern Iburi Earthquake detectedusing autocorrelation analysis of ambient seismic noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hiroki Ikeda and Ryota Takagi
Topographic effects on frequency-size distribution of landslides triggered by the Hokkaido Eastern Iburi Earthquake in2018 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mio Kasai and Takashi Yamada
Source rupture process of the 2018 Hokkaido Eastern Iburi earthquake deduced from strong-motion data considering
seismic wave propagation in three-dimensional velocity structure . . . . . . . . . . . . . . . . . . . . . . Kimiyuki Asano and Tomotaka Iwata
Characteristics of relocated hypocenters of the 2018 M6.7 Hokkaido Eastern Iburi earthquake and its aftershocks with athree-dimensional seismic velocity structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Saeko Kita
Characteristics of seismic activity before and after the 2018 M6.7 Hokkaido Eastern Iburi earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Takao Kumazawa, Yosihiko Ogata and Hiroshi Tsuruoka
Spatiotemporal crustal strain distribution around the Ishikari-Teichi-Toen fault zone estimated from global navigation
satellite system data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mako Ohzono, Hiroaki Takahashi and Chihiro Ito
The 2018 Hokkaido Eastern Iburi earthquake (MJMA = 6.7) was triggered by a strike-slip faulting in a stepover segment: insightsfrom the aftershock distribution and the focal mechanism solution of the main shock. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kei Katsumata, Masayoshi Ichiyanagi, Mako Ohzono, Hiroshi Aoyama, Ryo Tanaka, Masamitsu Takada,

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Teruhiro Yamaguchi, Kazumi Okada, Hiroaki Takahashi, Shin’ichi Sakai, Satoshi Matsumoto,
Tomomi Okada, Toru Matsuzawa, Shuichiro Hirano, Toshiko Terakawa, Shinichiro Horikawa,
Masahiro Kosuga, Hiroshi Katao, Yoshihisa Iio, Airi Nagaoka, Noriko Tsumura, Tomotake Ueno
and the Group for the Aftershock Observations of the 2018 Hokkaido Eastern Iburi Earthquake 10

Geodetically estimated location and geometry of the fault plane involved in the 2018 Hokkaido Eastern Iburi earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tomokazu Kobayashi, Kyonosuke Hayashi and Hiroshi Yarai
Source parameters and site amplifications estimated by generalized inversion technique: focusing on the 2018
Hokkaido Iburi-Tobu earthquake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kenichi Nakano and Hiroshi Kawase
Estimation of convergence boundary location and velocity between tectonic plates in northern Hokkaido inferred by
GNSS velocity data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chihiro Ito, Hiroaki Takahashi and Mako Ohzono
Electrical resistivity modeling around the Hidaka collision zone, northern Japan: regional structural background of the2018 Hokkaido Eastern Iburi earthquake (Mw 6.6) . . . . . . . . . . . . Hiroshi Ichihara, Toru Mogi, Hideyuki Satoh and Yusuke Yamaya
Three-dimensional S-wave attenuation structure in and around source area of the 2018 Hokkaido Eastern Iburi
Earthquake, Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ryoichi Nakamura and Takahiro Shiina
Estimation of the source process and forward simulation of long-period ground motion of the 2018 Hokkaido Eastern
Iburi, Japan, earthquake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hisahiko Kubo, Asako Iwaki, Wataru Suzuki, Shin Aoi and Haruko Sekiguchi
Coseismic changes in groundwater level during the 2018 Hokkaido Eastern Iburi earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tomo Shibata, Ryo Takahashi, Hiroaki Takahashi, Takanori Kagoshima,

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Naoto Takahata, Yuji Sano and Daniele L. Pinti 13

Cover image: Iwasaki, T. et al., Earth, Planets and Space 2019, 71:103 DOI: 10.1186/s40623-019-1081-z